Canon Bob wrote in post #17265778
It is.....especially for a wide zoom.
I'm not sure that I could be that bold when expressing my opinion about it.
What makes the TS-E's so great (and the Zeiss 21mm as well) is they have excellent even performance across the frame. Compared at f/8, the 16-35mm f/4L is slightly sharper in the center, but slightly less sharp in the corners. Still, look pretty close to me.
EF 16-35mm f/4L:
I also have been giving a lot of thought about my own wide-angle kit. I just added the Nikon 14-24mm to my bag because I feel there is goodness in having a UWA zoom alongside a pair of great primes. I consider my TS-Es essential. Sadly, that leaves no room for the awesome Zeiss primes. For that reason I sold my Zeiss 21mm earlier this year. Wish I could have it all, but for now I'm very pleased with a TS-E 17, 24 II, and 14-24 kit.
So, in the OP's case, consider this: sell the 15mm for the excellent 21mm (and keep the TS-E 17). Add the 16-35mm f/4L and the wished for focal length range is mostly covered, while also having two fantastic primes.
One more consideration: if the 15mm gets sold it will be sorely missed. I had the also-excellent 14L II and sold it because, as I mentioned before, 14-15mm is a challenging not-the-most-useful focal length. But, man, I really missed having a 14mm in my bag. So, I am elated about getting that focal length back in the much more useful 14-24mm zoom. So, my zoom suggestion is just about a way for the OP to have cake and eat it too.